The present invention relates to an image forming apparatus such as an electrophotographic copying machine and an electrostatic recording apparatus in which an electrostatic transfer process is used to form an image when a photoreceptor is charged.
In order to charge a photoreceptor such as a photoreceptor drum, a corona charger has been generally used hitherto, wherein high voltage is impressed upon a discharge wire and thereby a strong electric field is generated around the discharge wire for gaseous discharge. The photoreceptor is charged when electric charge ions generated in the course of gaseous discharge are adsorbed onto the surface of the photoreceptor.
A corona charger used in the conventional image forming apparatus mentioned above has an advantage that a photoreceptor is not damaged in the course of charging thereon because the charger does not come into mechanical contact with the photoreceptor. The corona charger, however, has a disadvantage, due to high voltage used therein, that there is a risk of an electric shock or electric leakage and also ozone generated in the course of gaseous discharge is harmful to human bodies and the ozone shortens the life of a photoreceptor. Further, charging voltage by means of a corona charger is sharply influenced by temperature and humidity to be unstable, and noise is caused by high voltage in the corona charger, which is a serious disadvantage on the occasion where an electrophotographic image forming apparatus is utilized as a terminal equipment for communication or an information processing apparatus.
These many disadvantages of a corona charger are caused by gaseous discharge necessary for charging.
Therefore, there are disclosed in Japanese Patent Publication Open to Public Inspection Nos. 133569/1984, 21873/1992 and 116674/1992 (hereinafter referred to as Japanese Patent O.P.I. Publication) the charging devices wherein magnetic particles are adsorbed on a cylindrical conveying carrier which is a charging roller holding therein magnetic objects for forming a magnetic brush, and the magnetic brush rubs the surface of the photoreceptor for charging it, as a charging device capable of charging the photoreceptor without conducting high voltage gaseous discharge carried out in a corona discharge and without giving any mechanical damages on the photoreceptor.
However, the charging devices disclosed in the patents described above have several problems. That is, according to the charging devices, it is impossible to charge a photoreceptor stably and uniformly. To be in more detail, in a transfer region, the magnetic particles on the surface of the cylindrical magnetic particle conveying carrier are formed chain-like along the lines of magnetic force, and charging is performed through these chain-like magnetic particles, however, unless the condition of magnetic particles in the transfer region is stable, charging is not uniformly performed, and dielectric breakdown of the photoreceptor occurs, and uneven charging is conducted. Further, discharge is caused between the magnetic particle chains forming the magnetic brush, so that ozone is generated. Further, when the charging operations are conducted over a long period of time, the magnetic particles are deteriorated. Therefore, after a certain period of time has passed, the charging condition is different from that in the initial copying stage.
In a magnetic brush charging operation, when a magnetic pole of the magnet 23 is arranged at a position where the magnetic pole is opposed to the photoreceptor 10, magnetic particle chains 121 are formed as shown in FIG. 5, and the photoreceptor 10 coming into contact with the magnetic particles is charged through these chains. When these magnetic particle chains are locally contacted with the photoreceptor 10, the contact portions are over-charged, and dielectric breakdown is caused in the photoreceptor, and uneven charging is caused.
As shown in FIG. 6, in a charging device of the prior art, a charging region is set up at a position where the charging roller 22, which is a rotational member to convey the magnetic particles 21, and the photoreceptor drum 10, which is a photoreceptor, are most closely located, and a magnetic pole 23a of the magnet 23 is provided at a position opposed to the aforementioned position where the charging roller 22 and the photoreceptor drum 10 are most closely located. The magnetic particles 21 in the charging region are connected along the lines of magnetic force of the magnet 23 so that they are formed into a chain-shape. In this way, bristles 21A of the magnetic brush are formed. When a bias voltage is impressed between the photoreceptor drum 10 and the charging roller 22, the photoreceptor drum 10 is charged through the conductive bristles 21A of the magnetic brush. As shown by small arrow marks in FIG. 6, electrical discharge occurs in the magnetic brush and between the chains of the magnetic particles 21, so that ozone is generated. When the magnetic particles 21 are used over a long period of time, they are deteriorated, and especially in the initial stage of a copy operation, the electrical resistance of the magnetic particles 21 is lowered, so that the electrical discharge is further facilitated.
As shown in FIG. 7, in the case where an amount of the magnetic particles 21 is small, the bristles 21A of the magnetic brush becomes rough. Therefore, the bristles 21A are not uniformly contacted with the surface of the photoreceptor 10. As a result, the photoreceptor 10 is locally overcharged, which causes dielectric breakdown of the photoreceptor drum 10 which is a photoreceptor, and further uneven charge occurs. In the case where an amount of the magnetic particles 21 is too much, the magnetic particles 21 are deposited on the surface of the photoreceptor drum 10, so that the charging region is unnecessarily extended, and further the electrical resistance of the bristles 21A is unnecessarily lowered and an over-current is caused. Further, the magnetic particles are not sufficiently oscillated, which causes uneven charging.
It is an object of the present invention to solve the problems described above and to provide an image forming apparatus in which dielectric breakdown of a photoreceptor can be avoided and generation of ozone can be prevented.